In recent years, as a high energy density battery, secondary batteries such as a lithium-ion secondary battery or a nonaqueous electrolyte secondary battery have been developed. The secondary battery is anticipated for use as a power source for vehicles such as a hybrid electric automobile and an electric automobile, or as a large-sized power source for power storage. When the secondary battery is used as the power source for vehicles, the secondary battery is demanded to achieve rapid charge/discharge performance and long-term reliability or the like in addition to the high energy density.
Rapid charge and discharge is enabled by lithium ions and electrons rapidly moving respectively through an electrolyte and an external circuit between a positive electrode and a negative electrode that are able to have lithium ions and electrons be inserted and extracted. The battery capable of performing rapid charge/discharge has the advantage that a charging time is considerably short. When the battery capable of performing rapid charge/discharge is used as the power source for vehicles, the motive performances of the automobile can be improved, and the regenerative energy of power can be efficiently recovered.
A carbon-based negative electrode using a carbonaceous material such as graphite as a negative electrode active material is used as a negative electrode, which can have lithium ions and electrons be inserted and extracted. However, when rapid charge and discharge is repeated in a battery including the carbon-based negative electrode, dendrites of metallic lithium may precipitate on the negative electrode. The dendrites of metal lithium may cause an internal short circuit. Therefore, when the rapid charge and discharge is repeated in the battery including the carbon-based negative electrode, a concern is raised that heat generation and ignition may occur.
Therefore, a battery including a negative electrode using a metal composite oxide as the negative electrode active material in place of the carbonaceous material has been developed. In particular, in a battery using a titanium oxide as the metal composite oxide for the negative electrode active material, the dendrites of metal lithium are less likely to precipitate even when rapid charge/discharge is repeated as compared with those of the battery including the carbon-based negative electrode. The battery using the titanium oxide has more stable rapid charge/discharge and a longer life than those of the battery including the carbon-based negative electrode.
However, the titanium oxide has a higher (more noble) potential relative to lithium metal than that of the carbonaceous material. On top of that, the titanium oxide has a lower theoretical capacity per unit mass than that of the carbonaceous material. Therefore, there is a problem that the battery including a negative electrode using the titanium oxide as the negative electrode active material has a lower energy density than that of the battery including the carbon-based negative electrode.
In view thereof, a new electrode material containing titanium and niobium has been considered. In particular, in a monoclinic niobium-titanium composite oxide represented by Nb2TiO7, while tetravalent titanium ions are reduced to trivalent titanium ions when lithium ions are inserted, pentavalent niobium ions are reduced to trivalent niobium ions, also. Therefore, this monoclinic niobium-titanium composite oxide can maintain the electric neutrality of a crystal structure even when many lithium ions are inserted, as compared with the titanium oxide. As a result, the monoclinic niobium-titanium composite oxide represented by Nb2TiO7 has a high theoretical capacity of 387 mAh/g.